The practice of giving food animals small doses of antibiotics has been around since at least the 1940s. In the earliest days they were used, these so-called growth promoters conferred huge benefits, sometimes doubling the weight of animals without requiring more feed.

But what if they no longer work? That’s the provocative question raised in a report by the Review on Antimicrobial Resistance, a British project that has been examining the best way to control antibiotic use on farms—and the antibiotic resistance that results—for more than a year.

The main thrust of the Review report released Monday in London is a proposal to set internationally agreed-to limits on how much on-farm antibiotics any country can use. This runs counter to control policies created by Europe and the